fix(harness): close the turn's event source when delivery stops early

[michaelxu2288]

Problem

The harness delivery adapters never close the event source they iterate:

• auto_send, the async push path behind UnifiedEmitter.auto_send_turn;
• yield_events and UnifiedEmitter.yield_turn, the sync HTTP yield path.

Their finally blocks close streaming contexts and flush span derivation, but leave turn.events alone. Every CLI tap depends on being closed. The Claude Code and Codex taps close their raw line iterator in a finally, and that close is what terminates the CLI subprocess in the scaffolds.

It goes wrong in two cases:

• a turn is cancelled (an interrupt, or an activity cancel) while delivery is awaiting the backend (ctx.stream_update or a context close) rather than the source;
• a sync client disconnects mid-stream.

In both, the source stays suspended at a yield. The turn object keeps a reference to its event generator, and agents hold the turn to read turn.session_id. So garbage collection never finalizes it, and the CLI subprocess and its stdout pipe leak.

Fix

• auto_send and yield_events close events in their finally, after the existing cleanup. The close is guarded with contextlib.suppress(Exception), so a failing aclose cannot mask the original exception.
• UnifiedEmitter.yield_turn closes its delivery generator explicitly instead of leaving it to async-generator finalization.
• Plain async iterators without aclose, and generators that are already exhausted, behave exactly as before.

Verification

• New tests:
  • test_auto_send.py: cancel auto_send while stream_update blocks, with the source held by a local and no gc.collect();
  • test_yield_delivery.py: close yield_events early;
  • test_emitter.py: close yield_turn early, with a turn that pins its generator the way the CLI taps do.
• All three fail on main with assert [] == [True] (the source's finally never ran) and pass here.
• An extra test checks that a source without aclose still delivers normally.
• uv run pytest -n 0 tests/lib/core/harness tests/lib/adk: 571 passed, 1 skipped.
• ruff check and pyright (1.1.399) are clean.

Confidence Score: 5/5

The PR appears safe to merge; no actionable issue remains established.

Summary

Harness delivery now closes event sources when delivery stops early, so CLI tap cleanup runs on cancellation and client disconnects. The ACP response stream also closes its handler generator, carrying disconnect cleanup upstream.

• Async push delivery closes its source after its existing cleanup.
• Sync harness delivery passes early closure through to the event source.
• ACP response streaming closes the handler generator when the stream ends.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  A[ACP response body closes] --> B[Handler generator closes]
  B --> C[yield_turn closes delivery]
  C --> D[yield_events closes turn events]

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